A laterite ore body is an oxidised ore, and a laterite ore body generally consists of a limonite upper layer (of the ore profile) and a saprolite lower layer. Geological studies have shown that the major nickel containing mineral in the laterite upper layer is the low magnesium content limonite and the major cobalt mineral is asbolane. The major nickel containing minerals in the lower saprolite layer are the high magnesium containing species, serpentine, chlorite, smectite and nontronite. The cobalt content of the saprolite layer is negligible. It must be noted that generally there is no clear demarcation between upper and lower laterite ore layers and on occasions an intermediate layer is often referred to as a transition zone.
In order to establish a sensible, cost effective treatment of a laterite ore body, all mineral types containing nickel and cobalt should be treated in a manner to recover maximum metal values in a simple single process without discarding damaging material to the environment. In this respect, a major consideration with respect to the environmental issues is the nature of the iron compound contained in the discarded ore tailings. A major consideration with respect to cost effective high metal recovery is the quantity and thus cost of the acid used in the leaching process.
Acid leaching of laterite ore solids whether by pressure treatment in an autoclave or by leaching at atmospheric pressure and elevated temperatures results in an acidic discharge which must be neutralised partially before the metal values can be recovered. At the high temperatures used in autoclave digests, typically around 250° C. to 275° C., acid consumption to dissolve metals is doubled or tripled due to the formation of the bisulfate ion (HSO4) and a single proton (H). This is illustrated in the following equations:MeO+2H2SO4→Me(HSO4)2+H2OMe2O3+6H2SO4→2Me(HSO4)3+3H2O
The reversion of the bisulfate to the sulfate ion occurs on cooling the slurry releasing an additional proton thus the cooled slurry inevitably contains excess acid which must be neutralised.
U.S. Pat. No. 4,548,794 (Californian Nickel Corporation) describes the use of the saprolite fraction of the ore to neutralise the acidity of the limonite pressure leach material. However the temperature of the neutralisation was high and the nickel and cobalt recoveries were low.
U.S. Pat. No. 6,379,636 (BHP-Billiton) describes a process which involves pressure acid leaching of limonite followed by atmospheric pressure leaching of saprolite using the autoclave discharge slurry in combination with selected alkali metal ions to form jarosite, M(Fe3 (SO4)2 (OH)6), M=Na, K, or NH4, in which form the iron is discharged to the tailings dams.
However iron discharged as jarosite results in high acid consumption as it is known that of the 1.5 moles of sulfuric acid required to dissolve 1 mole of ferric iron, only 1 mole of sulfuric acid is released during jarosite precipitation to aid leaching the saprolite fraction. Jarosite is not a stable compound and slowly releases acid as it weathers, which could have negative environmental impacts.
U.S. Pat. No. 6,391,089 (Curlook) describes a leaching process whereby the acidic autoclave discharge solution was recycled to the ore feed preparation stage thus effecting a significant reduction in acid consumption. However there are complications with the recycling of dissolved magnesium and excess sulfuric acid is needed for magnesium bisulfate Mg(HSO4)2 formation at the autoclave leach temperature.
Consequently a process which combines the benefits of high metal recovery, per mole of acid consumed, from the complete ore body and the discharge of environmentally acceptable waste solids is most desirable.
The present invention aims to provide a process which overcomes or minimises the difficulties associated with the prior art.
The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of this application.